The growing demand for electricity expected during the coming decades has increased interest in the development of new technologies for energy production from renewable power sources, such as wind and solar. However, the success of these new renewable power sources needs to be coupled with the introduction of competitive energy storage devices for load-leveling and peak-shaving such that these renewable sources could be tied to the grid. In this fashion, the problem of the unpredictable and intermittent energy production behavior of renewable power sources may be overcome. For electrical energy storage, electrochemical devices such as batteries and supercapacitors have been shown to provide higher efficiencies compared to other energy storage systems currently utilized.
Reduction-oxidation i.e. Redox Flow Batteries (RFBs) store electrical energy in a chemical form and subsequently dispense the stored energy in an electrical form via a spontaneous reverse redox reaction. A redox flow battery is an electrochemical storage device in which an electrolyte containing one or more dissolved electro-active species flows through a reactor cell where chemical energy is converted to electrical energy. Alternatively, the discharged electrolyte can be flowed through a reactor cell such that electrical energy is converted to chemical energy. The electrolytes used in flow batteries are generally composed of metal salts dissolved in a solvent that are stored in large external tanks and are pumped through each side of the cell according to the charge/discharge current applied. Externally stored electrolytes can be flowed through the battery system by pumping, gravity feed, or by any other method of moving fluid through the system. The reaction in a flow battery is reversible, and the electrolyte can be recharged without replacing the electroactive material. The energy capacity of a redox flow battery is therefore related to the total electrolyte volume, such as the size of the storage tank. The discharge time of a redox flow battery at full power also depends on electrolyte volume and often varies from several minutes to many days.
Within the wide variety of electrochemical devices for energy storage, redox flow batteries are one of the best options for massive storage due to their higher capacity in comparison with other battery technologies. RFBs typically employ two soluble redox couples at high concentrations in aqueous or organic media which are stored in two external tanks and pumped into an electrochemical reactor, where one of the species of the redox couple is transformed into the other, storing or delivering electrons depending upon whether the device is charging or discharging. The electrochemical reactor may be composed of a stack of two-electrode cells. The two electrodes are typically composed of graphite bipolar plates and carbon felts. These electrodes are separated by an ionic exchange membrane, such as Nafion, to avoid mixing of the positive and negative half-cell electrolytes.
Many types of RFBs have been widely explored since the first appearance of the Fe—Cr flow cell in 1973, including hybrid systems and chemically regenerative redox fuel cells. However, only the iron-chromium, all-vanadium (VRB), zinc-bromine and sodium-polysulfide (PSB) cells have come close to full-scale commercialization. At this point, the reduction of cost of the different materials employed in the electrodes, the membranes and the electrolyte is mandatory to promote the introduction of RFBs in the worldwide market.
All-copper redox batteries have been previously reported based on acetonitrile in the articles by B. Kratochvil and K. R. Betty, J. Electrochem. Soc., 121 (1974) 851-854 and P. Peljo, D. Lloyd, N. Doan, M. Majaneva, K. Kontturi, PCCP, 16 (2014) 2831-2835, ionic liquids in the article by W. W. Porterfield, J. T. Yoke, Inorganic Compounds with Unusual Properties, ACS Publications, Washington, D.C., p. 104,1976 and deep eutectic solvents in the article by D. Lloyd, T. Vainikka and K. Kontturi, Electrochim. Acta, 100 (2013) 18-23. However, the currents supported by these systems remain fairly low.
The article by L. Sanz, J. Palma, E. Garcia, M. Anderson, J. Power Sources 224 (2013) 278-284 discloses a study of the degree of electrochemical reversibility of the Cu(I)/Cu(II) redox couple in chloride media at 1M concentration of copper. Only the positive half-cell reaction is discussed. It was found that the values of peak potential separations of this couple were comparable to those displayed by vanadium redox couples, showing a quasirreversible behavior. In addition, a noticeable displacement of the formal potential of the Cu(I)/Cu(II) redox couple towards much more positive values was observed, reaching the experimental potential displayed by the Fe(III)/Fe(II) redox couple, which has also been widely employed in flow cells, for instance in a hybrid all-iron configuration and more recently in the Fe—V RFB.
The problem of low currents encountered in the known all-copper systems needs to be solved before the all-copper system is ready for industrial scale application. Moreover, the cross-contamination over the cell membranes arising from using dissimilar elements for the two electrode reactions is a concern as it degrades the stability and shortens the life cycle of the present RFBs.
There is still a need for an affordable industrial scale redox flow battery design which is able to provide technically useful energy efficiency while using cost-effective cell materials. Moreover, the operation of the battery should be environmentally and occupationally safe and readily up-scalable.